1. Field of the Invention
The present invention relates to an inductor used in various kinds of compact thin electronic equipments, and a method for producing the inductor, and more specifically, the invention relates to an inductor having a structure containing a coil conductor wound on a drum core and a composite magnetic material in a sheet form covering the outer periphery of the coil conductor, and a method for producing the inductor.
2. Description of Related Technology
An inductor using a drum core generally has a magnetic material covering an outer periphery of a coil conductor wound on the drum core in order to meet requirement on producing electronic equipments having a small size and a low profile. For example, JP-A-9-120926 proposes an inductor 110 shown in FIGS. 13A and 13B produced in such a manner that a coil wire 113 wound in a hollow shape is placed to straddle on a pair of lead frames 116a and 116b, a lead end and a tail end of the coil wire 113 are conductively connected to the lead frames 116a and 116b, respectively, and a magnetic powder molding material 111 containing magnetic powder and a resin is molded under pressure onto the periphery of the coil wire 113. JP-A-2001-185421 proposes a magnetic device 210 (220) shown in FIGS. 14A and 14B produced in such a manner that magnetic members 212a, 212b and 212c (222a, 222b and 222c) formed, for example, of a ferrite sintered body are placed in a drum form, a coil conductor 213 (223) is wound on the magnetic member 212c (222c) in a rod shape, and the members are embedded in a composite magnetic member 211 (221).
In recent years, there are increasing tendencies of decrease in size and thickness of electronic equipments. Accordingly, an inductor having a small size and a low profile is demanded for the electronic equipments. However, a composite magnetic material containing a resin and magnetic powder can provide an apparent magnetic permeability μ′ of only about 10. Therefore, there is such a problem that the inductor 110 disclosed in JP-A-9-120926, in which the circumferences, including the inner circumference, of the wire coil 113 in a hollow shape are surrounded with the magnetic powder molding material 111 having a low magnetic permeability cannot provide a high inductance unless the number of turns of the wire coil is increased.
In the magnetic device 210 disclosed in JP-A-2001-185421, the composite magnetic member 211 containing a magnetic powder and a thermosetting resin is adhered to the magnetic member 212 corresponding to the collar of the drum core. Accordingly, a large residual stress remains inside the magnetic member 212 due to hardening of the resin contained in the magnetic member 211, so as to have such a problem that the apparent magnetic permeability μ′ of the magnetic member 212 is decreased due to the presence of the residual stress to fail to provide a high inductance. Furthermore, there is another problem that a difference in linear expansion coefficient occurring between the composite magnetic member 211 and the magnetic member 212 formed, for example, of a ferrite sintered body induces cracks in the magnetic member 212 associated with an internal stress formed upon reflow soldering or a heat cycle test and the aforementioned residual stress.